If you have non-basic lands or "utility lands" (lands that do things other than mana) and you have Celestial Dawn out, do these lands become basic Plains and lose all their abilities?
They become plains, and do indeed lose all of their other abilities and simply tap for W. However they are not basic lands, basic is a supertype so if they are not basic lands to start with they don't gain that supertype simply because they become a basic land type.
If you have non-basic lands or "utility lands" (lands that do things other than mana) and you have Celestial Dawn out, do these lands become basic Plains and lose all their abilities?